The present invention relates to a process for hot forming precision structures in or on flat glass, especially plate glass, in which a heated forming tool is pressed with its structuring surface on one side of the flat glass. The invention also relates to an apparatus for performing that process.
Flat glass provided with large-scale precision structures is required for precision applications, especially in the optical glass field. This type of glass includes, for example display panels of new generation flat display screen devices (Plasma Display Panels (PDP), Plasma Addressed Liquid Crystal (PALC)). Micro-channel structures for control of individual lines or columns, which extend over the entire active display screen width or height and in which a plasma is ignited by electric discharge, are provided in this flat display screen glass. The boundary of an individual channel at both sides or ends is provided by a rectangular crosspiece whose width is as small as possible (i.e. &lt;100 .mu.m). In order to obtain a sufficient discharge volume, the height of the crosspiece is substantially larger than its width. The spacing of the crosspieces should be as small as possible. Currently typical values of between 360 .mu.m and 640 .mu.m are achieved in small scale production. The height of the crosspieces amounts to from about 150 .mu.m to 250 .mu.m at a width of from 50 .mu.m to 100 .mu.m.
During the structuring of this flat display screen glass, which for example is a 25"-PALC screen of a size of 360 mm.times.650 mm, the exact lateral dimensioning, relative positioning and reproducibility of the channel and thus the stability of the forming tool are crucial because of the later positioning of the electrodes. With a method based on hot shaping by means of a conventional Chromium-Nickel-Steel tool, the thermal expansion coefficient amounts to about 12.times.10.sup.-6 /K. For example, for a tool length of about 360 mm, as required for a 25"-PALC display screen, this always causes a length change of about 4 .mu.m per K temperature fluctuation. Considering that the required positioning accuracy of the electrodes in the micro-channels is in the range of .+-.10 .mu.m, a temperature fluctuation of .+-.2.5 K can cause considerable problems. The permissible temperature fluctuations are considerably reduced in the larger display screens, for example 42"-display screens.
The problems are similar in other applications of flat glass with precision structures.
Existing specifications limit however the possible applications of conventional hot forming methods, such as rolling or pressing. Two different process variants of the conventional hot forming methods exist.
The hot pressing occurs with very hot glass. The forming tool, a roller or press member provided with a suitable structure, is cooled, so that energy is drawn from the hot glass and solidification of the structure occurs.
In cold pressing an energy input occurs by means of a forming tool, which comprises a roller or a press member and is heated by a suitable energy source.
Conventional hot shaping methods have the following disadvantages:
When a contact between the glass and a press or roll tool acting as the forming tool occurs only for a short time, i.e. prior to solidification the work tool is removed from the glass, because of flow of the glass structure, a strong rounding occurs after this contact. PA1 In a long-duration contact which is used in a cold-pressing method, intolerable stresses arise because of strong temperature differences and different thermal expansions of the tool and glass. PA1 Since the forming tool is heated completely in a conventional hot shaping, in order to achieve a sufficient surface temperature on the contact surface for the glass, high non-reproducible temperatures occur in the required precision range of .+-.2 K (with typical work tool steels and glass surface areas required by the specifications), which lead to intolerable deformation of the work tool. PA1 A higher tool wear, which requires a replacement of the forming tool, occurs during the making of structured glass with reduced structure radii.
It is more difficult to prevent adherence of the tool to the glass in both methods with increasing tool temperature.
An additional essential requirement of the method of making these glasses is the maintenance of a stable production process, in which the local distribution and form of the structures are kept extremely constant. Additional limitations of the conventional hot forming are as follows:
The corresponding disadvantages are also present in the process described in DE 38 08 380 A1 for impressing programs on glass disks, in which a pre-pressed glass disk with a smooth surface is heated by a radiator plate in a limited surface region and, after that, is immediately provided with the desired surface structure by an impressing stamp.